1. Field of the Invention
The present invention relates to the structure of a sensor which undergoes a chemical or physical interaction with the object (amount) to be detected and which is adapted for use in chemical sensor devices for detecting chemical amounts of gases, humidity, ions, etc., biosensor devices for detecting physiologically active substances such as uric acid and glucose, or physical sensor devices for detecting physical quantities of electromagnetic waves, temperature, etc. More particularly, the invention relates to the structure of a sensor which has a surface shaped to a suitably predetermined optimum form by an artificial reproducible method such as a fine processing technique so as to exhibit improved performance.
The sensors embodying the present invention are not always limited to those acting to convert the object (amount) to be detected directly to an electric signal but also include those performing an indirect converting action, for example, by undergoing chemical or physical interaction with the object (amount) to convert the same to another chemical or physical quantity.
2. Description of the Prior Art
Gas sensor devices are adapted to detect a specified component gas of a gaseous mixture in terms of an electric signal, for example, by
(1) utilizing a phenomenon on the solid surface of a sensor resulting from the adsorption of the gas by the surface, PA1 (2) utilizing the reactivity of a sensor with the gas, PA1 (3) utilizing concentration polarization (electromotive force) due to a solid electrolyte, or PA1 (4) utilizing the physical properties (thermal conductivity, infrared absorption, etc.) of molecules of the gas.
Generally, semiconductor gas sensor devices are based on the principle that when gas molecules (or radicals) are adsorbed by the surface of an n-type or p-type semiconductor (sensor) composed chiefly of a metallic oxide such as tin oxide (SnO.sub.2), zinc oxide (ZnO), nickel oxide (NiO) or cobalt oxide (CoO), transfer of electrons or uneven presence of charges occurs between the semiconductor and the adsorbed molecules (radicals) to form a space-charge layer in the vicinity of the semiconductor surface, consequently varying the electrical conductivity of the semiconductor, the variation in the conductivity thus indicating the presence of the gas. For example, when a semiconductor of SnO.sub.2, ZnO or like metallic oxide exhibiting n-type conductivity adsorbs a combustible gas such as hydrogen, carbon monoxide or hydrocarbon, transfer of charges between the adsorbed gas molecules and the semiconductor (donation of electrons by the adsorbed gas molecules) gives increased electrical conductivity in the vicinity of the semiconductor surface. Thus, the variation in the electrical conductivity due to the adsorption of the gas is detectable as a variation in the surface electrical conductivity. This means that if the surface of the sensor is increased relative to its volume, for example, by reducing the thickness thereof, the variation ratio of the conductivity increases to render the sensor serviceable advantageously. However, many of actual semiconductor gas sensors are polycrystalline bodies obtained by sintering a powder and have in the body of the sensor a contact portion or neck portion between crystal grains. For example when there is a boundary 2 between crystal grains 1 in contact with each other as shown in FIG. 7, a space-charge layer 3 is formed over the surface of the grains exposed to the atmosphere, owing to the influence of adsorbed oxygen (electron acceptor), so that the two grains contact each other through the space-charge layer 3. Accordingly, an electron barrier indicated in a curve 4 is formed between the grains to impede the movement of electrons between the grains. It is thought that when a combustible gas comes into contact with the grains, the adsorbed oxygen is consumed or removed by combustion, lowering the potential barrier as indicated in a curve 5 in FIG. 7 and increasing the electrical conductivity. With the actual semiconductor gas sensor, the contact portion at the grain boundary contributes a great deal to the gas detecting mechanism, permitting the sensor to exhibit pronounced variations in the surface electrical conductivity. On the other hand, the output characteristics, i.e. the conductivity-gas concentration characteristics, of the semiconductor gas sensor are dependent on alterations in the minute structure of the contact portion or neck portion between the crystal grains. This is a great factor causing the characteristics to differ from sensor to sensor. Basically, therefore, it is necessary to control the size and shape of the grains and the state of fusion between the grains with good reproducibility. From the viewpoint of sensitivity to gas, it is also necessary to increase the area of adsorption of gas so that the space-charge layer formed in the vicinity of the sensor surface will greatly contribute to the conductivity as already described.
When sensors are to be prepared by a conventional method, for example, by sintering a powder, the sintering temperature and time, addition of the sintering agent, gaseous sintering atmosphere, etc. are controlled empirically by relatively controlling indirect conditions. Nevertheless, when such indirect condition control is resorted to, it is not always possible to control a single factor only. For example, depending on the sintering time or temperature, at least the size and shape of the crystal grains vary, and the state of fusion between the grains (state of the grains bound together) also alters. For this reason, it is extremely difficult to produce with good reproducibility sensors which are identical in structure when viewed on a microscale.
Next, conventional biosensor devices will be described. Generally, biosensor devices comprise a sensor called a receptor which is prepared by fixing a living body associated substance, such as an enzyme, antibody or organella of the living body, to a suitable substrate (film), and a transducer for converting to an electric signal a gas or like chemical substance or physical amount of light, heat or the like resulting from, or eliminated by, the reaction of the receptor with the substance to be detected. For example, in the case of sensor devices for detecting glucose, the receptor is prepared by fixing an enzyme (glucose oxidase, GOD) to a high polymer film. When glucose contacts the enzyme GOD, hydrogen peroxide (H.sub.2 O.sub.2) is formed according to the following reaction formula. ##STR1## The H.sub.2 O.sub.2 produced is detected in terms of an electric signal using electrochemical means (transducer) having, for example, a platinum anode. Thus, the concentration of the substance to be detected, i.e. glucose, can be determined by detecting the amount of the resulting H.sub.2 O.sub.2. While the chemical substance produced is detected by the transducer in this way, methods are also known of detecting emission of light or endothermic, exothermic or like thermal phenomenon resulting from the reaction between the enzyme and the substance to be detected. When the light emitting phenomenon is utilized, a photodetector is used as a transducer, while a thermistor or like temperature sensor is used for detecting the thermal phenomenon. In any case, the lower limit of the detectable concentration of the object substance is almost always dependent on the amount of reaction between the receptor and the substance. Accordingly, the sensitivity of detection is increased by fixing the living body associated substance, such as enzyme, with the highest possible density, or by increasing the area of contact of the receptor with the substance to be detected. However, the increase in the area of the receptor is physically limited because the enzyme or the like is fixed usually to a flat substrate and further because the size of the receptor needs to be considered relative to the size of the transducer.
Next, temperature sensors heretofore known will be described. Temperature sensors generally include those adapted to detect infrared rays, those utilizing the variation of electrical resistance with temperature and those utilizing thermal electromotive force. So-called thin film temperature sensors will be described below in which the temperature measuring element is a thin film of platinum, nickel or like material having a great temperature coefficient of resistance.
The temperature measuring elements for thin film temperature sensors must fulfill the requirements of having a great temperature coefficient of resistance which is constant over a wide temperature range, being low in resistance value at a reference temperature (e.g. 0.degree. C. or 100.degree. C.) so as to be interchangeable, having a resistance value in a readily usable range for temperature measurement (e.g. a resistance value R.sub.0 of 100 .OMEGA. or 1 k.OMEGA. at 0.degree. C.), being small, etc. Accordingly, the conventional temperature measuring element is produced by forming a thin film of platinum, nickel or like metal having a high purity on a substrate of ceramics or the like having a surface made planar to the greatest possible extent, by vacuum evaporation or sputtering, and thereafter forming the film into a pattern of lines having a specified width by photoetching or other technique. To give the temperature measuring element a minimized area and a resistance value R.sub.0 which is convenient to use, e.g. 100 .OMEGA. or 1 k.OMEGA., at a reference temperature (e.g. 0.degree. C.), the pattern is prepared in a zigzag (meandering) form with a minimized line width. However, because of the limitations actually involved in the processing technique especially when platinum is used, it is difficult to obtain a pattern up to several micrometers in line width and line spacing, consequently imposing limitations on minimizing the size of the temperature measuring element. With temperature measuring elements having a resistance value R.sub.0 of 1 k.OMEGA., the lower limit of the size of elements is approximately 1 to 2 mm square.